The unlimited recording and restoring of contents opens the possibility for an abusive use of these contents that are generally protected by copyright. Many protection and control means for restoring contents have been developed as shown in the examples below.
The document EP1169856B1 describes a method of accessing an encrypted content. The latter is protected in a local network by the re-ciphering of the key required for the decryption of the content. A first unit receives and stores the encrypted data content and encrypts the decryption key of the data with a key specific to this first unit. The data and the encrypted key are transmitted to a second unit belonging to the network for their decryption. According to an embodiment, the key associated to the first unit is a public key whose corresponding private key is located in the second unit.
The document U.S. Pat. No. 7,181,624 describes a device for recording and reproduction of digital data broadcast in a transport stream. The device appends temporal data indicating the arrival time to each data packet included in the transport stream. A block key is generated to encrypt a data block including several transport packets each accompanied with the arrival time temporal data of the packets. This key is created from a root block comprising additional information unique to the data block including the arrival time temporal data and encrypts each data block. The digital data thus encrypted are stored in the device memory. The reproduction of data is carried out according to a decryption process inverse to that of the encrypting during the recording.
The document US2004010467 describes a memory card for mobile equipment comprising a memory zone intended to store encrypted data content, a zone to store license data transmitted by a distribution system, a plurality of zones for storing authentication data authenticated by public keys common to the distribution system. In response to a distribution request coming from the mobile equipment, a license server verifies the authenticity of the memory card and distributes the encrypted content and the corresponding license to the mobile equipment. An encrypted content received by first mobile equipment can be transferred to second mobile equipment able to exploit the content only if the corresponding license has been acquired. Therefore the second equipment will have to obtain the license comprising a decryption key of the content from the distribution system which will verify the conformity of the memory card.